A 56-year-old smoker presented with a 3-day history of intermittent chest pain. Cardiovascular examination on admission was normal. ECG showed minimal (<1 mm) ST elevation in lead III, and right-sided leads revealed similar mild ST-elevation. Troponin I was elevated at 10.91. He was managed as a delayed-presentation ST-elevation myocardial infarction case. 12 h following admission, he developed oxygen-resistant hypoxia and hypotension. There were no clinical or radiological signs of pulmonary congestion. CT pulmonary angiogram revealed no pulmonary embolus. A bubble-contrast echocardiogram confirmed an active interatrial right-to-left shunt without Valsalva provocation. His hypoxia steadily improved over the following 24 h and he did not require any shunt closure. A repeat bubble contrast echocardiogram showed that the shunt was now only active following a Valsalva manoeuvre. Transoesophageal echocardiography confirmed a patent foramen ovale. This is a rare but important cause of resistant hypoxia following a right-heart infarct.